Problems arising when a light emitted from a semiconductor laser device is coupled to an optical fiber are the coupling efficiency and the alignment precision between the semiconductor laser device and the optical fiber. Since the beam divergence of an ordinary semiconductor laser device for optical communication is as wide as about 20.degree. to about 30.degree., only a very low coupling efficiency of several percents may be realized when the laser light is directly coupled to the optical fiber.
When a lens is inserted between the semiconductor laser device and the optical fiber, a high coupling efficiency can be obtained. However, the alignment precision will then be about 1 .mu.m, so that it becomes necessary to perform a highly precise alignment, thereby presenting a cost-increasing factor.
In order to solve this problem, a method has been devised to directly couple laser light to an optical fiber while narrowing the beam divergence of the semiconductor laser device to about 10.degree.. FIG. 1(a) shows an example of a conventional structure of a semiconductor laser device which realizes such a narrow beam divergence (H. Fukano et al., Electron. Lett., vol. 31, No. 17 pp. 1439-1440, Aug. 17, 1995).
The structure includes a stripe 101 having an active layer (hereinafter, referred to also as the "striped active layer 101") and an InP burying layer 102 surrounding the stripe. The striped active layer 101 includes a tapered region 103 and a parallel region 104. Laser light 105 is emitted from an end face of the tapered region 103.
For light propagating from the parallel region 104 to the tapered region 103 of the striped active layer 101, light confinement in the active layer 101 continuously decreases as the light propagates through the tapered region 103. Accordingly, leakage of light from the active layer 101 into the burying layer 102 increases, whereby the spot size of the laser light 105 at the emission end is enlarged with respect to the spot size in the parallel region 104. Such an enlargement of the spot size of the laser light 105 means narrowing of the beam divergence.
In the above-described conventional structure, the striped active layer 101 is divided into the parallel region 104 having a constant width and the tapered region 103 having a continuously-varying width. In such a structure, when the length of the tapered region 103 is relatively long, as shown in FIG. 1(b), the variation of stripe width is gentle, whereby a radiation mode has less influence on an emitted light pattern. Thus, for the laser light 105 emitted from the parallel region 104 via the tapered region 103, an emitted light pattern with a single peak, as shown in FIG. 1(c), can be obtained. However, since the total cavity length increases, in view of the operating characteristics of the semiconductor laser, problems arise such as an increase in the threshold current and a decrease in the slope efficiency. Moreover, the number of laser elements to be obtained from a substrate of the same size decreases, thereby increasing the production cost per element.
On the other hand, when the tapered region 103 is short, as shown in FIG. 1(d), the total cavity length decreases, but the influence of the radiation mode on the emitted light pattern becomes more significant, thereby resulting in an emitted light pattern with a plurality of peaks, as shown in FIG. 1(e). Thus, the coupling efficiency between the semiconductor laser and the optical fiber decreases.
In view of the above points, it is necessary to realize an emitted light pattern with a single peak at a narrow beam divergence in a semiconductor laser device without deteriorating the operating characteristics such as the threshold current and the slope efficiency.
The present invention, which has been made in view of such problems, has objectives of: (1) providing a semiconductor laser device with a low threshold current, a high slope efficiency characteristic and a narrow beam divergence; (2) providing a production method therefor; and (3) providing an optical communication system including such a semiconductor laser device as a light source.